netdev - Re: how to distinguish between PHYs that are not distinguished in the device tree?

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Date:   Wed, 12 Dec 2018 11:02:59 -0500
From:   rpjday@...shcourse.ca
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev@...r.kernel.org
Subject: Re: how to distinguish between PHYs that are not distinguished in
 the device tree?


Quoting Andrew Lunn <andrew@...n.ch>:

> On Wed, Dec 12, 2018 at 08:16:55AM -0500, rpjday@...shcourse.ca wrote:
>>  (i asked about this on the kernel newbies list earlier as a kind of
>> puzzle but i think i really want a definitive answer so i'm taking it
>> to the experts. (i will *try* to be concise.)
>>
>>   imagine i'm building a bunch of 8-port switches, using PHYs from
>> acme corp, who manufacture four different PHYs that differ only in
>> their supported speeds, with PHY IDs that differ only in the final
>> nybble which identifies the PHY speed:
>>
>>   0x1234.5671   1G
>>   0x1234.5672   2.5G
>>   0x1234.5675   5G
>>   0x1234.567A   10G
>>
>>   the problem i have is that i have been handed a bucket of mixed
>> PHYs of all four types that are visually indistinguishable -- the
>> only way to tell them apart is, upon system boot, i can read a given
>> H/W register which precisely identifies the type of PHY of the four
>> possible types. however, in this situation, all i can do is reach into
>> the bucket, grab eight random PHYs, bolt them in, and somehow later
>> dynamically identify them at runtime. how to do this?
>>
>>   first, i can't identify the precise PHY ID in the device tree file
>> since i have no idea what type will be at each of the eight PHY addresses,
>> so i will "wildcard" the compatible value for each of them thusly (where
>> the final nybble of zero is, of course, irrelevant):
>>
>>   compatible = "ethernet-phy-id1234.5670"
>
> You don't need to specify any compatible string in the device
> tree. The PHY subsystem looks at the ID registers to determine which
> PHY driver to load. You only need to use ethernet-phy-id1234.5670 when
> the PHY has the wrong ID in its registers.

   ah, that clarifies things quite a lot, thanks.

rday